This invention relates to vehicle exhaust mounted oxygen sensors of the solid electrolyte type and particularly to apparatus for measuring the internal impedance thereof during operation of the vehicle engine.
Oxygen sensors of the solid electrolyte type are used with vehicle mounted internal combustion engines as exhaust mounted sensors for providing an air-fuel ratio signal to the engine fuel supply apparatus in order to maintain maximum efficiency in a three-way catalytic converter, which requires the air-fuel ratio to be kept within very narrow limits. A sensor of this type normally contains a solid electrolyte cell which is effective to generate internally, in response to the composition of the exhaust gases to which it is exposed, an electrical voltage signal across a pair of output terminals. When the sensor becomes sufficiently warm, the voltage across the output terminals tends to vary only a little from a typical value of 100 millivolts when the air-fuel ratio is lean and a typical value of 900 millivolts when the air-fuel ratio is rich and to change rather sharply between 100 and 900 millivolts in a narrow region around stoichiometry.
The temperature of such sensors has a great effect upon their performance characteristics. For instance, a sensor at a temperature far below the "warm sensor" temperature has an extremely high internal impedance and thus a voltage across its output terminals determined by any external voltage source connected across the terminals. In addition, even at temperatures above the "warm sensor" temperature, such sensors often exhibits a change in slope with temperature in the output voltage curve within the fast changing region about stoichiometry which results, in the dynamic operation of a closed loop fuel control system, in a sensor response time which correspondingly decreases with increasing temperature.
In addition, the hostile environment of the hot, corrosive and fast flowing exhaust gases causes wear on the portion of the sensor exposed to such gases, which wear may result over time in an increase in the response time of the sensor at a given temperature, as well as an increase in the internal impedance of the sensor at that temperature. It has been found that restoration of the internal impedance of the sensor to the original internal impedance of the sensor when new, such as by using heating means to increase the temperature of the sensor until the desired internal impedance is reached, may have the beneficial result of restoring the original response time. Thus, the internal impedance of such a sensor is found to be a good indication of sensor temperature in the short run and of sensor response time in the long run. It would therefore be advantageous in many closed loop fuel control systems using such sensors to provide apparatus for measuring the internal impedance of an oxygen sensor without interfering in the control of the fuel system by the sensor.
A well known technique for measuring the internal impedance of a battery cell involves the steps of first measuring the output voltage of the unloaded cell and next measuring the output voltage of the cell when loaded by a known test impedance. The ratio of said measured voltages varies with the internal impedance of the cell, showing a very small value when the cell internal impedance is large and a value increasing toward one as the cell internal impedance becomes smaller. In order to apply this technique to the case of an oxygen sensor in a vehicle engine closed loop fuel control system, switching means must be provided for automatically switching an internal impedance into and out of a series circuit relationship with the oxygen sensor. For the practical reasons of size, cost and reliability, such switch apparatus is preferably in the form of a semiconductor switch in an electronic circuit.
There is a problem, however, with using a semiconductor switch in such a circuit with a typical oxygen sensor. A semiconductor switch is characterized when conducting by an impedance which may increase greatly at very small conduction currents. When the sensor is warm, there is generally sufficient voltage generated across the sensor terminals to produce a current through the test impedance and switch large enough that the test impedance predominates. However, a cold sensor generates a very small voltage across its terminals, due to the high internal impedance, and thus generates very little current through the test impedance and switch. The switch impedance may thus become large enough to significantly affect the measured voltage across the sensor terminals and change the desired relationship between the loaded-unloaded sensor voltage difference and sensor impedance.